Deposition of aggregated protein into neurofilament-rich cytoplasmic inclusion bodies is a common cytopathological feature of neurodegenerative disease. How-or indeed whether-protein aggregation and inclusion body formation cause neurotoxicity are presently unknown. Here, we show that the capacity of superoxide dismutase (SOD) to aggregate into biochemically distinct, high molecular weight, insoluble protein complexes (IPCs) is a gain of function associated with mutations linked to autosomal dominant familial amyotrophic lateral sclerosis. SOD IPCs are detectable in spinal cord extracts from transgenic mice expressing mutant SOD several months before inclusion bodies and motor neuron pathology are apparent. Sequestration of mutant SOD into cytoplasmic inclusion bodies resembling aggresomes requires retrograde transport on microtubules. These data indicate that aggregation and inclusion body formation are mechanistically and temporally distinct processes.F amilial amyotrophic lateral sclerosis (FALS), a dominantly inherited form of ALS, is a progressive paralytic disorder resulting from the degeneration of motor neurons in the cortex, brainstem, and spinal cord (1, 2). Between 10% and 20% of FALS cases are because of missense mutations in the SOD1 gene, which encodes the cytoplasmic metalloenzyme Cu, Znsuperoxide dismutase (SOD) (3). Mice expressing human FALS-linked SOD1 transgenes develop an age-dependent ALSlike disorder characterized by profound degeneration of spinal motor neurons and by the presence in surviving motor neurons of neurofilament-rich cytoplasmic inclusions resembling pathological inclusion bodies in spinal motor neurons in human ALS and FALS. The highly penetrant dominant inheritance pattern of both the human and mouse diseases (4), together with the absence of motor neuron disease from ''knockout'' mice lacking endogenous murine SOD1 (5), strongly suggests that FALS pathology is because of a toxic gain-of-function in SOD. The biochemical nature of this toxic gain of function, however, and the mechanism by which SOD mutations cause the degeneration of motor neurons have remained elusive, largely because of the failure to identify novel properties of mutant SOD that are unambiguously linked to early cytopathological changes.One hypothesis argues that toxicity results from the tendency of mutant SOD to ''aggregate'' into cytoplasmic inclusion bodies (6) that are evident in motor neurons from SOD transgenic mice (7,8) and in cultured COS cells (9) or motor neurons (10) expressing mutant SOD cDNA. Cytoplasmic inclusion bodies are a hallmark of motor neuron degeneration in ALS and, indeed, of nearly all neurodegenerative diseases (11). SOD is itself a component of inclusion bodies in degenerating spinal cords from FALS patients (12, 13) and in end-stage mice expressing human FALS-linked SOD1 transgenes (6,7,12). How these inclusion bodies could cause neuronal degeneration-and indeed whether the inclusions are cytotoxic or even, perhaps, cytoprotective-is controversial. It has been suggested that ne...